1. Field of the Invention
The present invention generally relates to methods of and apparatus for use in producing a crystal sheet from a melt of semiconductor or metal, and in particular to methods of and apparatus for use in producing a sheet of silicon to be used as the substrate of a solar cell.
2. Description of the Background Art
Conventionally, a substrate of crystallized silicon is produced by producing an ingot in the Czochralski method or casting a source material to produce an ingot and then slicing the ingot for example with a wire saw. However, the slicing step is costly and cutting a portion results in a loss of the source material of silicon. As such, in the area of solar cell, to address an important issue, i.e., cost reduction, silicon ribbon methods are being increasingly developed. In this method, a sheet of silicon is extracted directly from a melt of silicon to eliminate the necessity of providing a slicing step.
Of such silicon ribbon methods, a method of growing a crystal having a large solidification interface is disclosed for example in Japanese Patent Laying-Open No. 61-275119. FIG. 49 schematically shows a method of producing silicon in the silicon ribbon method disclosed in the document. With reference to FIG. 49, a rotative cooling element 902 in the form of a cylinder has a side surface partially immersed into melted silicon 903, and rotative cooling element 902 is rotated and a silicon ribbon 901 solidified on a cylindrical surface of cooling element 902 is successively extracted. Note that melted silicon 903 is held in a container 904.
Japanese Patent Laying-Open No. 10-29895 also discloses a silicon-ribbon production apparatus. FIG. 50 is a schematic view of the silicon-ribbon production apparatus disclosed in the above publication. As shown in the figure, this apparatus is configured of a rotative cooling element 931 in the form of a cylinder, a container 934 holding melted silicon 932 therein, and a roller 935 guiding a silicon ribbon 933.
Rotative cooling element 931 has an cylindrical side surface partially immersed into melted silicon 932. As rotative cooling element 931 is rotated silicon ribbon 933 solidified and grown on the cylindrical surface of the cooling element is extracted successively.
Furthermore, a crystal sheet can be produced directly from a melt in an EFG (Edge-defined Film-fed Growth) method, in which a die having an opening in the form of a slit is used to raise a melt through capillarity and at an upper end of the melt a seed crystal is used to extract a silicon ribbon. A crystal sheet can also be produced in the Dendrite Web method, in which a melt has a surface supercooled to produce a crystal sheet.
In methods using a rotative cooling element in the form of a cylinder, however, silicon solidifies and grows to cover an exterior of the cylinder and silicon that is grown thus has a curvature along the cylinder and thus curves. Such silicon is inconvenient if it is used as a substrate of a solar cell as the substrate is required to be flat in a process step such as screen-printing an electrode, laminating, vacuum-chucking and the like. Furthermore, a conventional substrate tray provided to be suitable for a flat substrate, cannot be used. Furthermore, when grown silicon removed from a rotative cooling element is extracted successively in a predetermined direction it needs to be pulled in the direction with tensile strength precisely controlled. Furthermore, the sheet of grown, crystallized silicon warping in geometry is hardly pulled successively in one direction.
Furthermore, in the EFG method and the Dendrite Web method, a crystal sheet is grown at a rate significantly affected by heat of solidification generated and heat transfer determined by a temperature profile in a vicinity of a solid-liquid interface between the crystal sheet and the melt. As such, successively and reliably producing a crystal sheet entails precisely controlling the temperature of the solid-liquid interface and the temperature profile in the vicinity thereof. Current temperature controlling systems, however, would not satisfactorily respond in proportion to crystal-growth rate in general. Furthermore, in the above methods a crystal sheet that is being grown is cooled by annealing or through natural heat liberation. As a result, the crystal sheet is disadvantageously required to grow at a reduced rate.
Furthermore, when a silicon ribbon is removed from the rotative cooling element the exact silicon ribbon pulls and thus removes the subsequent silicon ribbon from the rotative cooling element. As such, an enormous load is imposed on the silicon ribbon and thus tends to damage the silicon ribbon. Furthermore, if a silicon ribbon is damaged extracting it can not immediately be resumed and it can thus hardly be reliably successively extracted.
Furthermore, in the conventional methods, it is difficult to control an in-plane temperature profile of a crystal sheet and it is thus necessary to consider thermal conductivity and the like in selecting a material for a substrate and a member therearound and also to optimize a heating portion such as a heater or a cooling portion in arrangement and geometry. In selecting a member, however, not only its thermal conductivity but its wettability and removability with respect to a crystal sheet, coefficient of thermal expansion matched, refractoriness and durability as well as cost need to be considered. As such, the temperature profile can hardly be optimized. Furthermore, it is often difficult technically as well as mechanically to optimize the heating portion and the cooling portion for both of the quality of the crystal of the crystal sheet and the removability thereof. In particular, removability is a parameter significantly depending on the material(s) of the substrate and precise control can thus hardly be achieved. As such, it is difficult to produce a crystal sheet of high quality successively and reliably.
The present invention has been made to overcome such disadvantages as described above. The present invention contemplates a method of manufacturing a crystal sheet capable of reliably, successively extracting a crystal sheet, an apparatus for use in manufacturing the same, and a solar cell employing the crystal sheet.
The present invention also contemplates a method capable of producing a crystal sheet of high quality, an apparatus for use in manufacturing the same, and a solar cell employing the crystal sheet.
The present invention provides an apparatus for use in producing a crystal sheet including: a plate having a main surface on which a crystal sheet is to be formed; a container holding a melt therein; a movable member holding the plate to bring the main surface of the plate into contact with the melt and then move the plate away from the melt; and cooling means for cooling the movable member.
In the apparatus thus configured a crystal sheet is formed on a main surface of a plate held by a movable member cooled by a cooling means. As such, the crystal sheet formed on the main surface of the plate can be cooled via the movable member with an optimal rate to provide a crystal sheet of high quality. Furthermore, the crystal sheet formed on the main surface of the plate can be free of warpage and it can thus be produced with high quality.
Furthermore, preferably, the apparatus for use in producing a crystal sheet further includes a removal means removing a crystal sheet from the main surface of the plate transported from the movable member. The plate is provided with a throughhole and the removal means has a first protrusion fit into the throughhole.
As such, the first protrusion of the removal means can be fit into the throughhole of the plate to remove the crystal sheet from the plate.
Furthermore, preferably, the movable member has a second protrusion fit into the throughhole of the plate. As such, if a crystal sheet is grown on the main surface with the throughhole receiving the second protrusion, any crystal is not formed in the throughhole. As such, the throughhole can be used to facilitate removing the crystal sheet from the main surface of the substrate.
Furthermore, preferably, the second protrusion has a top surface substantially level with the height of the main surface when the second protrusion is fit into the throughhole. As such, there can be produced a crystal sheet having a flat surface.
Still preferably, the first protrusion is larger in length than the second protrusion. This can ensure that a crystal sheet grown on the plate can be removed by the first protrusion
Furthermore, preferably, the plate and the second protrusion are formed of different materials. For example, the plate can be formed of a material allowing a crystal sheet to be readily removed therefrom and the second protrusion can be formed of a material allowing the crystal sheet to be readily grown thereon so as to readily remove the crystal sheet from the plate.
Furthermore, preferably, the removal means is provided with the first protrusion in a direction in which the plate moves and the movable member is provided with the second protrusion in a direction in which the plate moves. As such, rotating the movable member and the removal means to fit the first and second protrusions into throughholes allows a crystal sheet to be produced efficiently
Furthermore, preferably, the apparatus for use in producing a crystal sheet further includes a guiding member in the form of a belt guiding the plate from the movable member to the removal means. The guiding member connects a plurality of plates in the form of a caterpillar. As such, the guiding member can successively guide the plates to efficiently produce a crystal sheet. Furthermore, the guiding member connected to form a caterpillar allows repetitive growth and removal of crystal sheets
Furthermore, preferably, the plate has opposite ends each provided with a raised portion provided with a connection for connecting adjacent plates to the guiding member.
Furthermore, preferably, the apparatus for use in producing a crystal sheet further includes a guiding member in the form of a rail guiding the plate from the movable member to the removal means. The guiding member in the form of a rail allows the plate to be transported from the movable member to the removal means to efficiently produce a crystal sheet. Since the guiding member is provided in the form of a rail, the plate moving thereon can be moved smoothly.
Furthermore, preferably, the movable member is provided in the form of a polygonal prism. As such, the polygonal prism can have each plane brought into contact with each substrate to efficiently produce a crystal sheet.
Furthermore, preferably, the movable member and the plate are unleveled to allow them to be fit into each other. As such, the movable member and the plate can contact with each other over an increased area and the movable member can thus efficiently cool the plate.
Furthermore, preferably, the main surface of the plate is flat. As such, there can be produced a crystal sheet having a flat surface.
Furthermore, preferably, the main surface of the plate is unleveled. As such, there can be produced a crystal sheet having an unleveled surface.
Furthermore, preferably, the cooling means is provided internal to the movable member. As such, the apparatus can be miniaturized and the cooling means can also efficiently cool the movable member.
The present invention provides the method of producing a crystal sheet including the steps of: bringing a main surface of a cooled plate into a melt; moving away from the melt the main surface of the plate brought into contact with the melt, to solidify the melt on the main surface to grow a crystal sheet on the main surface; and removing the crystal sheet from the plate.
In the method thus configured a crystal sheet is solidified and grown on a main surface of a plate. This can facilitate controlling the temperature at the main surface of the plate to produce a crystal sheet of high quality. Since a crystal sheet is grown on the plate it does not have curvature and a crystal sheet in the form of a plate can thus be readily produced
Furthermore, preferably, the step of removing includes guiding a crystal sheet grown on the plate to output the sheet from a heating chamber and thus remove and collect the sheet.
Furthermore, preferably, the step of moving includes using a movable member to bring the main surface of the plate into contact with the melt and then move the plate away from the melt.
Furthermore, preferably, the step of bring includes bringing into contact with the melt the plate provided with a throughhole and the step of removing includes using a removal means provided with a first protrusion to be inserted into the throughhole of the plate from a side of the plate opposite to the main surface of the plate with a crystal sheet thereon to remove the crystal sheet from the plate. As such, the throughhole can receive the first protrusion to facilitate removing the crystal sheet from the plate.
Furthermore, preferably, the removal means is rotatable, the removal means is provided with more than one first protrusion in a direction in which the removal means rotates, and the step of removing includes rotating the removal means to fit more than one first protrusion into at least one throughhole of each of more than one plate to remove the crystal sheet from the plate. As such, the removal means that rotates can successively fit the first protrusion into the throughhole to efficiently remove the crystal sheet from the plate.
Furthermore, preferably, the step of bringing includes using the movable member provided with a second protrusion to bring the plate into contact with the melt with the movable member having the second protrusion fit into the throughhole. Since the plate is brought into contact with the melt with the second protrusion fit into the throughhole, the melt does not enter the throughhole and any crystal sheet is thus not formed in the throughhole. As such, a crystal sheet can be readily removed.
Furthermore, preferably, the movable member is rotatable, the movable member is provided with more than one second protrusion in a direction in which the movable member rotates, and the step of bringing includes rotating the movable member to fit each of more than one second protrusion into each throughhole of more than one plate to bring the plate into contact with the melt As such the movable member can be rotated to fit each of the second protrusions into a throughhole to efficiently bring the plate into contact with the melt
Furthermore, preferably, the step of moving includes solidifying a melt of silicon and thus growing a sheet of crystallized silicon.
Still preferably, a solar cell in accordance with the present invention is fabricated with a crystal sheet manufactured in any of the methods as described above.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.